Methods of conducting seismic exploration include the production and transmission of seismic waves through the earth's surface and the positioning of geophone receivers at strategic locations in the area of interest for receiving the direct or reflected signals. These signals are then correlated with the source of the seismic waves in an effort to determine the characteristics of the earth's strata in the area of interest.
Explosive charges and large vibrator trucks have been employed as commercial seismic sources for many years, but they have several disadvantages, including the unpredictable characteristics of the explosive sources. In recent years, interest has concentrated on the development of controlled, sweptfrequency seismic sources that can be used downhole with appropriate receivers positioned either in adjacent holes or on the surface. Early downhole sources suggested designs where the source of the vibratory power, be it pneumatic, hydraulic, electrical, or mechanical, was located on the surface and was somehow transmitted to a downhole actuator. Typical of these types of devices are those found in the teachings of the patents to Malmberg, U.S. Pat. Nos. 3,221,833; Miller, 3,282,371; Fair, et al., 3,718,205; and Brooding, et al., 3,909,776.
Other downhole seismic sources have been described for transmitting variable frequency seismic shear waves into earth strata formations surrounding a borehole. Such a system, for example, is described in the patent to Hardee, et al., U.S. Pat. No. 4,805,727.
Recent downhole seismic source designs have concentrated on either simplifying or eliminating the complex downhole supply piping. See, for example, the above-cited patent to Hardee, et al. and their CIP application filed Feb. 17, 1989, which proposes a downhole pneumatically-powered seismic source that is self-contained and operates off standard 7-conductor logging cable which is readily available in the logging industry. The system described in these references includes a seismic source that either carries a compressed or liquified gas charge, or else uses an internal tank system which can be recharged downhole between seismic runs by means of a small (1/2 horsepower or less) electric motor which can be supplied through the relatively small conductors in the standard 7-conductor logging cable. However, the pneumatically-powered seismic sources contain disadvantages including a limitation on the type of excitation waveforms that can be generated.
Paulsson, in U.S. Pat. No. 4,702,343, proposes a downhole hydraulic-powered seismic source that is continuously supplied with 15 horsepower of electrical energy through a special cable that carries instrument lines and a nominal 15 kw electrical power line. However, hydraulic-powered seismic sources have heretofore been limited by the requirement that they be supplied with a large amount of electrical horsepower energy from a power source remaining on the surface. Hence, such hydraulic-powered seismic sources need to be used with a special cable to transmit the great amount of electrical energy down to the seismic source in the borehole.